@kinther It is called "Blossom End rot". It is a common problem with tomatoes. It is because of either lack of calcium or irregular watering. If you have calcium deficiency, you can solve it by using egg shells, bake them for a few minutes in the oven at 120 deg. C, crush them and add some vinnegar to it and let it sit over night. Then use the "juice" and mix it with 10 times it's volume water. Water your plants every other day with it and it fixes it.

Looks like everyone already diagnosed for you, so I'll recommend another solution. Cal-mag is a decent product that will help correct this.

Blossom end rot possibly! Need more calcium in the soil if so! Some tomatoes are also prone to it, unfortunately.

How do the leaves look like?

I also suspect blossom end rot, but most soils have enough calcium, especially if you water with tap water.

Often, this is caused by a lack of airflow. If the plant can't transpire, not enough Ca is transported into the tips, which results in said symptoms.

Maybe remove some leaves and add a ventilator to give them some air current

Going to agree with the other posts here, I experienced the same thing on my first wave this year. We had an abnormally wet early summer and nothing in my garden was a fan. Sometimes, shit just happens.

As others said, blossom rot. Just hit it with some fertilizer (with calcium) and make sure you are water consistently. I do the fertilizer every 2 weeks or so.

Also pull off any that have that dark scab or mushy end. The insides will be exposed and probably not good to eat. You don’t want your plant wasting energy growing rotten tomatoes

Across the various sub's i've seen this question maybe 6 times in the past 2 months.

It must be tomato season.

Blossom end rot. Most common tomato disorder. Add more calcium to your soil; fastest way would be to go buy some fertilizer with calcium specifically, and if you have more time, to add some crushed limestone or shells to your soil. Bone meal is the most easily applied, fastest way to fix a calcium issue.

End rot. I got this on a few of my pepper plants while I was out of town for 3 weeks, and wasn't able to properly re-fertilize the soil with plant food, and the plants were not consistently watered.

Aah. Tomato crunchy bits.

Yes BER. Amish trick is to spray the tomatoes with milk - any dairy milk. It can be old or spoiled. The calcium in the milk is a rapid solution. It is also supposed to stop fungus problems too.